its not overheating but i loose water. have to add a half gallon or so a week. i think the radiator pushs it out the overflow but never sucks it back. when i stoped the car tonite i heard it bubling in the overflow. it had a steady flow of air comming out the vent tube. added a gallon of water. i think someone had put stop leak in this car at one time cause theres a stopleak like residue in there.

you guys think its a bad thermostat and radiator cap? or something serious? hopefully not a cracked head. like i said it doesn't indicate overheating, but the guage is a factory one could be off?

Stopleak? Maybe a head gasket leak in the past?

Any white smoke while driving? This would lend to a head gasket leak. Also, what type of antifreeze are you running? I know there were a lot of people who ran the newer extended life antifreeze, and found that the chemical composition allowed it to seep through traditional gasket materials. Sound like you're loosing more than that could cause but.

Also sounds like some compression getting into the coolant, maybe????? Small piece of head gasket blown out????

Our cars did not come stock with an overflow, so if an overflow tank is added, the radiator cap must be replaced with an overflow style cap. The overflow tank must also have the end of the hose submerged in coolant for it to be effective, and if new it will need to "burp" the air from the hose the first time the engine is heated and cooled.

With the engine cool, remove the radiator cap. Start the engine and run until warm. Use a bright flashlight to look for bubbles in the radiator tank. If there are bubbles you have compression gasses leaking into the cooling passages.

If not, look for discolored, crusty, stained or soft spots on the radiator core, hoses, and water pump.

One other trick for coolant problem diagnosis is to get a pressure tester. If you have a small leak, it may only leak at temperature (with pressure) and then evaporate so you never see it. I scored a used pressure tester off e-bay. That way it will leak cold and you can find where it's coming from.

this is on my "back and forth to work car" the is no white smoke. i also notice if i check in the morning the upper radiator hose is sucked flat. if you crack the radiator cap,it un flattens. i hope its not a head gasket. i tried to clean out the coolant at the car wash but i think the stop leak or whatever is in there just dilutes and never comes quite clean, i use to goto the car wash or use the garden hose stick it in the radiator and let the car run till the water was clear.

i also just been puttin water back in so maybe it is just boiling out?

i think i will try the radiator cap and thrmostat first since the cap looks kinda crusty. it still springs by hand.

You have the wrong radiator cap, or a bad cap. Select one for a mid 70s Firebird and the overflow will work properly.

You also need to run a proper 50/50 coolant blend.

Possibly you didn't properly purge the air from the system initially. You could possibly leave enough air in there for 1 to 2 gallons. This will cause air pockets and high pressure steam which will bleed off as it would be under higher pressure than a completely liquid closed loop. Jack the front end up with the cap off and start car. Allow to heat up until you see the coolant coming from the upper hose. Add coolant mix slowly until no air remains.

Since the upper hose is sucked flat, I believe you have an air in system issue or a very small leak only leaking under high pressure, very unlikely on the latter.

JV. I'm working on a similar problem right now. I had a 160 t-stat in and when I went somewhere the coolant would bubble up in the overflow tank after shutdown. I changed to a 180 stat and it stopped. Recently I changed back to the 160 and it started doing it again. Rad cap is the type to be used with a catch can and it's new. It never gets too hot even when bubbling over(I've got two gauges and neither gets over 200) so I know it's not over heating. Just some sort of pressure thing. I thought about trying a cap with less of a pressure rating(I have a 16# now)but that seems counterproductive. I think I'll put the 180 back in and see. I've been trying different fans out lately so I have other things going on at the same time.
If your upper hose is sucked flat it sounds like you have a bad cap.

Robert

it was a bad cap....gummed up with stop leak

Had to revive this old topic.
I changed back to the 180 t-stat. No bubbling up in the overflow after shutdown. I would rather run the 160 but it's embarrassing pulling into a show, shuting the car down and having it puke all over the ground. I might go buy a 13 or 14 # cap(instead of the 16) and try that with the 160 again. See if that makes any difference.
Just a thought...maybe the 160 closes to soon after shutdown and the back pressure against the rad forces the water to erupt out the cap with nowhere else to go? Or maybe the opposite.
Robert

180 is better than 160....cant remember if I have 180 or 190 in mine...think its 180...

'None' is good for me. The only time I have trouble is when it's cold outside the heater doesn't get as hot. That's not a big problem for me.

Drill a 1/8" hole in the thermostat where it will not interfere with the gasket seal or the spring. It will prevent abrupt closure.

Have you followed Banshee's advice on how to purge your system of air?

Maybe your bypass hole from the intake to the timing cover is plugged?

Jim, I hadn't thought of that but that still is below the t-stat in the system. I can't imagine that passage being clogged but if there was enough junk in there to do that, I'd have bigger problems. Beside that wouldn't explain why it is fine now.

Robert

Ahh. It may be below the thermostat but think about how the radiator connects with the timing cover and then also to the waterneck. See the circle? It's easy to get it plugged with permatex when you are installing the intake and/or timing cover. And it could be fine now for the same reason you stated earlier about the 160 stat closing quicker.

Jim, I thought about that a little bit more and decided that's probably not the case because when I shut it down it would bubble over right away and I always checked the thermometer to see if the temp was too high. It never was but it was high enough for the thermo to still be open(around 200. temp always goes up some after shutdown). If anything I guess it would stay open long after it stopped puking. Back to square two. I know when it happens, just don't know why.
Vikki, I can't say that I went to any great pains to purge the system of air but you would think that after running it for awhile air would be forced through into the radbefore too long anyway.
Robert